I've been to both many times. 18 hours isn't terribly long. A little more info will help. What are you looking for? Dinner and sleep? A night out? Quiet in general? What time do you arrive / depart - makes a huge difference given how terrible traffic is there.

Both treat Plats well in my opinion though I think the benefits at the W go further but not enough to make a difference on an 18 hour stay.